1. Field of the Invention
The invention relates to a method and apparatus for the vacuum coating of webs having a three-dimensional surface structure.
2. Discussion of Related Art
DE-AS No. 1 059 739 discloses a method in vacuum coating whereby heat-sensitive bands are passed through the coating zone on an accompanying cooling film and winding this cooling film, together with the bands in some cases to prevent the individual layers of the bands from sticking to one another. In this manner, however, coating on two sides is possible only by separating the tape and cooling film and then turning the band. The known method, however, is not sufficient when webs having an open structure in depth are involved, such as foams, nonwovens, fabrics etc., are to be coated with a relatively great amount of the coating material per unit of area.
In the case of vacuum coating by a physical deposition process such as vapor depositing and cathode sputtering, the substrate (the web of material) is subjected to great thermal stress, due to the unavoidable thermal radiation and to the heat of condensation that is released by the deposition. Since in the vacuum it is impossible to cool with gases or ambient air, the substrate is generally guided within the coating zone over a cooled surface, which is also described in DE-AS No. 1 059 739. This measure, however, promises to be successful only if the substrate has a good thermal conductivity and/or a slight thickness, and lies smoothly against the cooling surface, which is true to some extent for thin plastic films. The circumstances are quite different in the case of materials with three dimensional surface structure, i.e., in the case of foams, nonwovens, fabrics etc., which have pores, cavities, projecting surface particles etc., which cannot be laid smoothly against cooling surfaces, and which as a rule are poor thermal conductors and have a relatively great thickness. The coating material condenses preferentially on projecting particles of the irregular surface, where it yields its condensation heat in addition to the radiant heat. Removal of heat to the side and through the back of the substrate is negligible. The consequence is a rapid destruction of the substrate, even at low rates of deposition per unit area of the substrate. Conditions are still more unfavorable when the material is coated simultaneously from both sides, since in this case the thermal stress is doubled and no cooling can be performed in the coating zone.
Any attempt to divide the material into several batches and apply it to the web in a number of successive passes has also failed, since the roll of the material web is an ideal heat accumulator, so that the substrate temperature rose accumulatively in each pass and the substrate was destroyed in the second, or certainly at least in the third pass.
The invention is therefore addressed to the problem of providing a coating method and a coating apparatus of the kind described above, by which webs of material with an open structure in depth can be coated without thermal damage.
The solution of the problem described is achieved according to the invention, in the method described above, by the measures given in the specific part of claim 1, and in the apparatus described above.
By the movement back and forth of the material web, in an oscillatory operation, so to speak, combined with the rolling up each time of the repeatedly recooled cooling film, the establishment of a temperature intermediate between material web temperature and cooling film temperature, the "outwinding" of the heated cooling film, and the transfer of the heat by means of the cooling film to a heat transfer agent, it becomes possible to eliminate the build-up of heat in the material web and in the roll, so that the total amount of coating material to be applied to a corresponding number of portions per unit area can be divided up, and the number of passes or coating procedures can be increased virtually at will, with a correspondingly reduced temperature rise per individual pass.
With the method of the invention it is possible to coat the material web in both directions of movement, and even on both sides simultaneously, without unacceptable temperature increase, thereby increasing accordingly the capacity of the apparatus and hence the economy of operation.